The goal of this project is to determine the function of the AXH (ataxin-1/HBP1) domain, the single conserved region of the protein ATAXIN1 (ATXN1), and the role of this domain in pathogenesis of spinocerebellar ataxia type 1 (SCA1). SCA1 is an autosomal dominant neurodegenerative disorder characterized by progressive ataxia, motor impairment and death due to degeneration of cerebellar Purkinje cells (PCs) and brainstem neurons. It is caused by expansion of CAG repeats that encode a polyglutamine tract in ATXN1. Protein-protein interactions and protein sequences outside of the glutamine tract are essential for pathology. The function of the AXH domain is of particular interest because expression of Drosophila ATXN1 (dATXNI) that mainly consists of an AXH domain (but not a polyglutamine tract) induces phenotypes similar to those caused by expressing polyglutamine-expanded human ATXN1 (hATXN1[82Q]) in a fly model of SCA1. Our lab has shown that the AXH domain mediates both biochemical and genetic interactions important for SCA1 pathogenesis. I hypothesize that many of the toxic gain-of-function features that occur due to expansion of the glutamine tract in SCA1 are due to altered function of the AXH domain. Potential AXH domain interactors have been identified via the yeast 2-hybrid (Y2H) assay. Binding between full-length clones of interactors and the AXH domain will be validated using pull-down assays. Whether differential interactions occur between interactors and wild-type ATXN1 and hATXN1[82Q] will be investigated. Whether interactors compete with each other for binding to regions within the AXH domain will be determined. The effect of AXH-interacting proteins on hATXN1[82Q]-mediated cell death will be evaluated in a cell based assay. AXH-interactors that modify hATXN1[82Q]-mediated death in cell culture will be further studied in vivo by testing if mutants of their orthologs can modify the ATXN1 expression phenotypes in a fly model of SCA1. Using column chromatography we have identified proteins that form stable complexes with ATXN1. Interactors that modify the hATXN1[82Q] toxicity in Drosophila will be evaluated to determine whether they are in a stable complex with ATXN1 in mouse cerebellum. Depending on the availability of reagents, one AXH-interactor will be further studied to determine how such interactions might affect the function of the complex in wild-type and Ataxinl 154Q/+ knock-in mice. Spinocerebellar ataxia type 1 (SCA1) is a devastating neurodegenerative disease with no effective therapy. This study will enhance our knowledge of the disease by elucidating how proteins that bind to a conserved domain in ATXN1, the protein mutated in SCA1, alter the extent of toxicity of mutant ATXN1.